The present invention relates to digital communication systems, and more particularly to a method and apparatus for receiving multiple asynchronous signals through a digital link.
Digital signal transmission is used in a variety of applications. One application is the transmission of digitized television signals over a cable television network. The transmission of digitized television signals is particularly applicable in an optical fiber cable television distribution system.
The replacement of coaxial cable with optical fiber transmission lines in television distribution systems has become a high priority. Production single mode fiber can support virtually unlimited bandwidth and has low attenuation. Accordingly, a fiber optic distribution system or a fiber-coax cable hybrid would provide substantially increased performance at a competitive cost as compared to prior art coaxial cable systems.
One problem in implementing an optical fiber distribution system for cable television signals is to provide an economical means for multiplexing a plurality of channel signals for transmission. Normally, transmission of multiple signals in a time-domain multiplex system requires that the digitizing rates for the incoming signals not only be equal, but fixed in phase relationship from one signal to the next. For multiple source complex signals such as asynchronous video feeds (e.g., television signals formatted in accordance with the National Television Standards Committee "NTSC" standards), this requires the use of a frame synchronizer for each television channel signal. Each frame synchronizer is locked to a master time base, that is directly related to the transmission rate so as to prevent logic race conditions, signal skewing and other problems. Such a system of synchronizing all input signals is cumbersome, equipment intensive, and expensive.
It would be advantageous to provide a system for transmitting and receiving multiple asynchronous digital signals that does not require the use of frame synchronizers for each channel. Such a system would save cost, power, hardware requirements and result in reduced system maintenance requirements. The present invention provides a system having these and other advantages.